US8748832B2 - Ionizing radiation detection device with a semi-conductor detector having and improved spectrometric response - Google Patents

Ionizing radiation detection device with a semi-conductor detector having and improved spectrometric response Download PDF

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US8748832B2
US8748832B2 US13/535,852 US201213535852A US8748832B2 US 8748832 B2 US8748832 B2 US 8748832B2 US 201213535852 A US201213535852 A US 201213535852A US 8748832 B2 US8748832 B2 US 8748832B2
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pulse
processing means
digitized
detection device
analogue
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US20130168562A1 (en
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Andréa Brambilla
Patrice Ouvrier-Buffet
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Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
    • G01T1/16—Measuring radiation intensity
    • G01T1/24—Measuring radiation intensity with semiconductor detectors
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
    • G01T1/16—Measuring radiation intensity
    • G01T1/24—Measuring radiation intensity with semiconductor detectors
    • G01T1/241—Electrode arrangements, e.g. continuous or parallel strips or the like
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
    • G01T1/16—Measuring radiation intensity
    • G01T1/24—Measuring radiation intensity with semiconductor detectors
    • G01T1/247—Detector read-out circuitry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
    • G01T1/29—Measurement performed on radiation beams, e.g. position or section of the beam; Measurement of spatial distribution of radiation
    • G01T1/2914—Measurement of spatial distribution of radiation
    • G01T1/2921—Static instruments for imaging the distribution of radioactivity in one or two dimensions; Radio-isotope cameras
    • G01T1/2928—Static instruments for imaging the distribution of radioactivity in one or two dimensions; Radio-isotope cameras using solid state detectors

Definitions

  • the present invention relates to an ionizing radiation detection device with a semi-conductor having an improved spectrometric response.
  • These ionizing radiation detection devices are in particular applicable for non-destructively inspecting materials, searching for dangerous or illicit substances, for example in luggage. The materials or luggage to be inspected are then placed between the detection device and an ionizing radiation source. Another application could be medicine and observation of living organisms.
  • These ionizing radiation detection devices allow to image objects or living organisms to be inspected
  • An ionizing radiation detection device with a semi-conductor detector includes a detector 1 for ionizing radiation 5 of semi-conductor material which cooperates with an electronic circuit 2 for reading signals provided by the semi-conductor detector 1 .
  • FIG. 1 can be referred to.
  • the ionizing radiation detector 1 includes a crystal 1 . 1 of semi-conductor material having generally a parallelepiped shape two generally opposite main faces 1 . 2 , 1 . 3 of which, carry each one or more electrodes.
  • the semi-conductor material crystal has generally a thickness between a few hundred micrometers and a few millimeters, or even a few centimeters and an area of a few square centimeters or even a few tens of square centimeters.
  • the semi-conductor material can be for example CdZnTe, CdTe, HgI 2 , GaAs, Si and the ionizing radiations 5 can be alpha, beta, X, gamma rays or even neutrons.
  • Neutrons are not directly ionizing radiations but they induce ionizing radiations by particulates created when they interact with matter.
  • a first face 1 . 2 includes one or several electrodes called cathodes 3 . 1 to be brought to a first potential and the other face 1 . 3 includes one or several electrodes called anodes 3 . 2 to be brought to a second potential higher than the first potential.
  • the second potential is substantially the ground and the first potential is negative. Consequently, the reading circuit 2 can be connected to the anodes 3 . 2 avoiding any high voltage problem.
  • reading electrodes it will be meant those connected to the reading circuit.
  • Electrodes 3 . 1 , 3 . 2 are also used to bias the detector 1 so as to allow migration of carriers (that is electrons or holes) into the semi-conductor material, the carriers being created in the semi-conductor material under the effect of an interaction between the semi-conductor material and the incident ionizing radiation 5 . Indeed, an incident ionizing radiation having a sufficient energy will pull out electrons from the atoms of the semi-conductor material it collides with.
  • Electrons are collected by anodes 3 . 2 , that is why the latter are connected to the reading circuit 2 .
  • Cathodes 3 . 1 have generally a role restricted to biasing the detector 1 and a single cathode is frequently used.
  • anodes 3 . 2 arranged as an array or a strip, arranged in studs and isolated from each other are generally used.
  • Each anode 3 . 2 cooperates with a volume V of semi-conductor material which is facing it. This volume is represented by the hatched area in FIG. 1 . It actually makes up a pixel. Anode 3 . 2 is considered as associated with the pixel in the following.
  • the ionizing radiation 5 which passes through the object 7 is attenuated at the time when it reaches the detector 1 .
  • the intensity of the ionizing radiation 5 which reaches the detector 1 depends on the chemical composition and the density of the object 7 therethrough.
  • Reading electrodes 3 . 2 connected to the reading circuit 2 provide signals the intensity of which depends on the attenuation delivered by the object 7 . By processing these signals in the reading circuit 2 , a transmission contrast image can be obtained from the object 7 , which enables information about the inner structure of the object 7 to be gained.
  • Dual energy or multi-energy irradiation techniques enable information about the chemical composition and more details about the density of materials making up the object to be acquired.
  • the detector 1 To perform images of the observed object 7 with an acceptable quality, it is necessary for the detector 1 to have as good as possible spectrometric response, which necessitates a great number of pixels available and thus a great number of reading electrodes 3 . 2 . Then, each of them should be connected to a unit reading circuit. And yet, it appears that reading electrodes collect interfering signals that should be rejected if the desired quality is to be achieved.
  • the reading circuit 2 should then be capable of getting rid of these interfering signals.
  • Electronic noise is due to random fluctuations in the signal in the absence of exposure to ionizing radiations.
  • Induction share occurs when an ionizing radiation is absorbed in the vicinity of a pixel and charges generated by this radiation are not collected by this pixel. In such a case, they induce on the electrode associated with this pixel a current pulse immediately followed by another current pulse of the opposite sign.
  • Charge share occurs when an ionizing radiation is absorbed in a pixel or between two neighbouring pixels and the charges are collected by the two electrodes associated with these two neighbouring pixels.
  • FIG. 1 schematically represents the reading circuit 2 . It is comprised, for each reading electrode 3 . 2 , of a charge preamplifier 2 . 10 followed by a shaping circuit 2 . 11 .
  • a charge preamplifier 2 . 10 followed by a shaping circuit 2 . 11 .
  • Several shaping circuits 2 . 11 can be used, they can be derivating and integrating filters, Gaussian filters, trapezoidal or triangular filters. Most often, these filters are tunable and their time constant can be set. Generally, these filters are means enabling to perform an analogue pulse the amplitude of which is proportional to the charge collected by the reading electrode.
  • each comparator compares the signal provided by the filter with a determined threshold and the counter is implemented if the threshold is exceeded. This allows a coarse classification of the absorbed ionizing radiation in some energy channels. This type of circuit does not provide for processing interfering signals.
  • reading circuits of which further include an analogue converter in each pixel as the Timepix circuit developed by the Medipix collaboration or a digital-to-analogue converter and a time-digital converter circuit as the ERPC circuit from AJAT Company.
  • Both reading circuits measure the amplitude of pulses for each photon that interacted in the detector. They enable the energy spectrum of interactions which occur in each pixel of the detector to be obtained. They also are capable of summing the amplitude of pulses simultaneously detected in two neighbouring pixels to correct effects of the charge share. On the other hand, they cannot distinguish pulses due to induction share and process them as useful pulses as soon as their amplitude exceeds the detection threshold.
  • One purpose of the present invention is actually to provide an ionizing radiation detection device which corrects the effects of induction share and possibly charge share.
  • the obtained device enables a better spectrometric response to be obtained, that is a more reliable estimation, than in prior art, of the energy of interactions, by removing noise generated by induction share and possibly charge share.
  • the present invention provides an ionizing radiation detection device including a detector of semi-conductor material intended to be biased thanks to electrodes, among which reading electrodes connected to a reading circuit which digitizes signals they provide, these signals being processed to reject those causing a poor spectrometric response.
  • the present invention is an ionizing radiation detection device comprising a detector of semi-conductor material intended to be biased thanks to electrodes, among which reading electrodes capable of collecting charges created in the detector during an interaction between the ionizing radiation and the semi-conductor material of the detector and which are connected to a reading circuit including:
  • the first processing means can include analogue processing means including a charge preamplifier possibly connected to a shaping circuit, these analogue processing means being capable of providing an analogue pulse the amplitude of which is proportional to the charge collected by one of the reading electrodes.
  • the first processing means can include digital processing means capable of providing a digitized pulse when a charge has been collected by one of the reading electrodes, this digitized pulse being formed by a succession of discrete signals, these digital processing means being downstream of the analogue processing means insofar as the analogue processing means are present.
  • the digital processing means can include a digitizer and possibly downstream of it, digital shaping means.
  • the second processing means are digital.
  • They can further include means for determining a time deviation between two digitized pulses provided by the first processing means and coming from two neighbouring electrodes, and means for rejecting one of the digitized pulses and for preserving the other digitized pulse depending on its time deviation.
  • the second processing means correct a charge share.
  • the time deviation can be the deviation between the maximum of one of the pulses and the maximum of the other of the pulses, this deviation having to be included in a predetermined acceptance window for the rejecting means to reject one of the digitized pulses and to preserve the other.
  • the rejecting means can reject the digitized pulse the maximum amplitude of which is the smallest and preserve the digitized pulse the maximum amplitude of which is the greatest.
  • the second processing means can further include means for correcting the digitized pulse preserved together with the rejected digitized pulse.
  • the means for correcting the preserved digitized pulse can add to the maximum amplitude of the preserved digitized pulse the maximum amplitude of the rejected digitized pulse.
  • the second processing means or first processing means can further include amplitude discriminating means capable of rejecting any pulse that said processing means have to process having a maximum amplitude lower than a predetermined threshold. Thus, the electronic noise is removed.
  • the parameter determined by the second processing means can be a time parameter selected from the rise time of the pulse, the time elapsed between the start of the pulse and the first zero crossing of the pulse.
  • amplitude value of the pulse after a baseline crossing for example a zero crossing
  • This amplitude value of the pulse after a baseline crossing between the start and the end of the digitized pulse can be the minimum Min(i) of the pulse.
  • said time parameter can correspond to the time during which the analogue pulse has a positive amplitude.
  • the present invention also relates to an ionizing radiation detection method by a detector of semi-conductor material intended to be biased thanks to electrodes, among which reading electrodes capable of collecting charges created in the detector during an interaction between the ionizing radiation and the semi-conductor material of the detector and which are connected to a reading circuit, wherein the method includes the steps of:
  • the providing step by the first processing means can provide an analogue or digitized pulse.
  • the processing by the second processing means can consist in determining a time deviation between two digitized pulses provided by the first processing means and coming from two neighbouring electrodes, rejecting one of the digitized pulses and preserving the other digitized pulse depending on this time deviation. A correction of charge share is thus achieved.
  • processing by the second processing means can consist in correcting the preserved digitized pulse together with the rejected digitized pulse.
  • the method can additionally include an amplitude discriminating step for rejecting any pulse to be processed by the first processing means or the second processing means having a maximum amplitude lower than a predetermined threshold. A removal of electronic noise is thus achieved.
  • FIG. 1 illustrates the ionizing radiation detection principle using an ionizing radiation detection device according to prior art
  • FIG. 2A illustrates the appearance of the signal at the output of the charge preamplifier and FIG. 2B the appearance of the signal at the output of the shaping circuit, these signals coming from two neighbouring reading electrodes;
  • FIG. 3A shows a first example of the ionizing radiation detection device object of the present invention and FIG. 3B shows another example of the ionizing radiation detection device object of the invention;
  • FIG. 4A illustrates a digitized pulse coming from a reading electrode i and a digitized pulse coming from a neighbouring reading electrode i+1 and FIG. 4B shows the digital derivative of the preceding digitized pulses;
  • FIG. 5 is a diagram of an algorithm implemented in the second processing means illustrated in FIG. 3A ;
  • FIGS. 6A , 6 B and 6 C show exemplary analogue pulses digitized by a digital oscilloscope and provided by three neighbouring reading electrodes corresponding to different interaction situations between a ionizing radiation emitted by a cobalt 57 ionizing radiation source and a cadmium telluride semi-conductor detector;
  • FIG. 7A is the spectrum obtained with the same source-detector assembly as in FIGS. 6A-6C without induction share processing
  • FIG. 7B is the spectrum obtained with the same source-detector assembly with induction share processing
  • FIG. 8 illustrates the spectrum obtained with the same source-detector assembly as in FIGS. 6A-6C with only the processing of electronic noise, the spectrum obtained with the processing of electronic noise and charge share, the spectrum obtained with the processing of electronic noise, charge share and induction share.
  • the inventors have determined three types of interfering signals which degrade the performances of the ionizing radiation detection device.
  • FIG. 1 is again referred to. Induced charges appear at a reading electrode 3 . 1 , 3 . 2 due to the migration of carriers created by interaction between the ionizing radiation 5 and the semi-conductor material of detector 1 . Pulses obtained due to induction share are not caused by charges collected as a result of the interaction of the ionizing radiation 5 with the semi-conductor material detector but by the migration, movement of carriers in the semi-conductor material biased following the interaction.
  • the curve C 1 represents, as a function of time, the amplitude of a pulse provided by a charge preamplifier connected to a first reading electrode, due to the collection of charges generated by an interaction in the semi-conductor material.
  • This pulse is at first of a high amplitude, it has a quite quick rise time and then it slowly decreases.
  • the curve C 2 represents, still as a function of time, the amplitude of a pulse provided by a charge preamplifier connected to a reading electrode neighbouring to the first reading electrode.
  • the pulse is generated by induction shares, the latter being generated by the migration of carriers in the vicinity of this electrode.
  • the pulse has a much lower amplitude, it has quick rise time and fall time.
  • FIG. 2B is represented a pulse provided by the filtering means of the reading circuit which, in the example shown, are a delay line filter.
  • the curve C 1 ′ corresponds to the curve C 1 after filtering. It assumes the shape of a substantially symmetrical one-pole pulse and the duration of which is substantially equal to twice the delay brought about by the delay line. This delay is about 50 ns in the described example.
  • the amplitude of the one pole pulse is proportional to the charge released into the semi-conductor material of the detector through the interaction.
  • the curve C 2 ′ corresponds to the curve C 2 after filtering by filtering means 2 . 11 .
  • a positive component also called positive lobe
  • a negative component also called negative lobe
  • the sum of the signals provided by the neighbouring reading electrodes of a first reading electrode corresponds in principle to the charge generated during an interaction in the volume of semi-conductor material associated with this first reading electrode.
  • FIG. 3A is referred to.
  • the semi-conductor detector 1 provided with its electrodes 3 . 1 , 3 . 2 on two main faces, preferably opposite ones, is substantially identical to that of FIG. 1 . It is assumed that reading electrode 3 . 2 are anodes.
  • the detection device also includes a reading circuit 2 connected to each reading electrode 3 . 2 .
  • This reading circuit 2 includes first processing means 2 . 3 , connected to each reading electrode 3 . 2 , which are capable of providing a pulse when a charge has been collected by one of the reading electrodes 3 . 2 .
  • These first processing means 2 . 3 include in the example of FIG. 3A analogue processing means 2 . 1 including a charge preamplifier 2 . 10 connected to each reading electrode 3 . 2 and an analogue shaping circuit 2 . 11 connected at the output of each charge preamplifier 2 . 10 .
  • a charge preamplifier 2 . 10 is a means well known to those skilled in the art and it is not described more in detail.
  • the analogue shaping circuit 2 . 11 can be a delay line filter or the like.
  • the shaping circuit 2 . 11 is, in this example, a delay line filter. It includes in a first branch, a delay line 21 connected to the charge preamplifier 2 . 10 on a first side, and to the unity gain amplifier 22 on the other side.
  • the output of the unity gain amplifier 22 is connected to a negative input of a substractor 23 , this substractor 23 having a positive input connected to the output of the charge preamplifier 2 . 10 .
  • it is substracted from a signal provided by the charge preamplifier 2 .
  • the output of the substractor 23 supplies an amplifier 24 with a gain higher than one.
  • the output of the amplifier 24 with a gain higher than one provides an analogue pulse the amplitude of which is proportional to the charge collected by the reading electrode to which the first processing means 2 . 3 are connected. This collected charge is generally proportional to the energy released by the ionizing radiation that interacted with the volume of semi-conductor material.
  • the shaping circuit 2 . 11 enables the one-pole and generally symmetrical pulse to be obtained.
  • This pulse can for example be in the form of a Gaussian pulse.
  • the analogue processing means 2 . 1 could only include the charge preamplifier 2 . 10 and not the shaping circuit 2 . 11 .
  • the first processing means 2 . 3 further include, downstream of the analogue processing means 2 . 1 , digital processing means 2 . 2 comprising a digitizer 2 . 20 of the analogue pulse provided by the analogue processing means 2 . 1 .
  • This digitizer 2 . 20 is formed by as many analogue-digital converters as shaping circuits 2 . 11 . Each analogue-digital converter is connected at the output of a shaping circuit 2 . 11 .
  • This digitizer 2 . 20 converts the analogue pulse provided by the analogue processing means 2 . 1 , in the example by the shaping circuit 2 . 11 into a digitized pulse.
  • a digitized pulse is formed by a plurality of successive discrete digital signals, represented by points in FIGS. 4A , 4 B as will be seen subsequently.
  • FIG. 4A is represented a digitized pulse coming from a reading electrode i and a neighbouring reading electrode i+1, during an interaction of the ionizing radiation in a volume of semi-conductor material associated with the reading electrode i.
  • References i and i+1 correspond to the row of the reading electrode on a line of successive reading electrodes.
  • the digitized pulse coming from the reading electrode i is positive, which means that the analogue processing means 2 . 1 also provide a positive analogue pulse. But the invention is also naturally applicable to the case where the pulse corresponding to a measured charge is negative.
  • the reading circuit 2 further includes second means 2 . 4 for processing pulses provided by the first processing means 2 . 3 , which are intended to reject any pulse which would reflect an induction share.
  • the second processing means 2 . 4 are digital, they are located downstream of the digital processing means 2 . 2
  • the second processing means 2 . 4 can be formed by a microprocessor, for example, a programmable logic circuit such as a FPGA (field-programmable gate array).
  • the outputs of all the analogue-digital converters 2 . 20 are connected at the input of the microprocessor 2 . 4 .
  • the second processing means 2 . 4 have the output thereof connected to means 2 . 5 for operating digitized pulse which have not been rejected by the second processing means 2 . 4 , these digitized pulses are called “preserved” ones.
  • Such operating means 2 . 5 include for example a microprocessor, for example that of a user device UD, such as a personal computer. These operating means 2 .
  • spectrometry line enabling an energy spectrum of non-rejected (or preserved) digitized pulses to be obtained. They can also be made by an imager each pixel of which represents the number of digitized preserved pulses in one or several energy bands, these digitized pulses being generated by a determined anode.
  • the second processing means 2 . 4 will carry out processing on pulses coming from first processing means 2 . 3 , so as to reject pulses which would not be useful signals, but interfering signals created by induction share and possibly charge share and/or electronic noise.
  • the spectrometric response of the ionizing radiation detection device is improved.
  • the digital processing means 2 . 20 can comprise downstream of the digitizer 2 . 20 , a shaping circuit 2 . 110 which then would be digital. It is shown in dotted lines in one of the paths of FIG. 3A and the analogue shaping circuit is represented crossed.
  • the shaping circuit 2 . 110 would process a digitized pulse, that is made up of discrete digital signals.
  • the shaping circuit 2 . 110 can comprise a delay line, an amplifier as well as a substractor which are arranged as described above for the analogue shaping means 2 . 11 . Its operation is similar to that previously described, except that the processed pulses are digital and no longer analogue.
  • analogue processing means 2 . 1 would only comprise the charge preamplifier 2 . 10 .
  • the reading circuit 2 includes, in the example described in FIG. 3A , downstream of the electrodes 3 . 2 , first processing means 2 . 3 for providing a pulse when a charge has been collected by one of the reading electrodes, these first processing means 2 . 3 including analogue processing means 2 . 1 for delivering an analogue pulse when the charge has been collected, this analogue pulse having an amplitude proportional to the charge collected by the reading electrode and following means 2 . 2 for digitally processing said analogue pulse, including a digitizer 2 . 20 which delivers a digitized pulse;
  • second means 2 . 4 for processing the pulse delivered by the first processing means 2 . 3 to reject if it is suspected to correspond to an induction share and possibly a charge share or electronic noise.
  • FIG. 4A represents, as a function of time, with reference C 3 , a digitized pulse coming from a first reading electrode i.
  • This digitized pulse is formed by a succession, in time, of discrete digital signals.
  • This digitized pulse has the greatest amplitude. This maximum amplitude corresponds to the maximum value of the discrete digital signals making it up.
  • this digitized pulse is a one-pole pulse, it does not cross the baseline which corresponds here to the zero value.
  • the reading electrode can be arranged as a strip or an array.
  • the electrodes neighbouring an electrode i will be the reading electrode i ⁇ 1 which precedes it and the electrode i+1 which follows it.
  • an electrode i When it is an array, an electrode i is located between an electrode i ⁇ 1 and electrode i+1 of the same line. It neighbours successive electrodes h ⁇ 1, h and h+1 which are in the line preceding the line wherein the electrode i is located. It also neighbours successive electrodes j ⁇ 1, j and j+1 which are in the line which follows the line wherein the electrode i is located. Electrodes h ⁇ 1, i ⁇ 1, j ⁇ 1 are in a same column, electrodes h, i, j are in a same column, electrodes h+1, i+1, j+1 are in a same column.
  • the second processing means 2 . 4 are digital and are capable of rejecting a digitized pulse which corresponds to electronic noise.
  • the second digital processing means 2 . 4 include means 20 for discriminating in amplitude discrete digital signals of a digitized pulse delivered by the first processing means 2 . 3 .
  • These discriminating means 20 include means 200 for determining an electronic noise discriminating threshold S_max i for the discrete digital signals of a digitized pulse. They also include means 201 for rejecting any digitized pulse of which no discrete digital signal would exceed the electronic noise discriminating threshold S_max i and for preserving in the opposite case.
  • FIG. 4A is represented the electronic noise discriminating threshold S_max i and it can be seen that the digitized pulse C 3 exceeds the electronic noise discriminating threshold S_max i at least locally. The digitized pulse C 3 will be preserved.
  • the electronic noise is removed from the useful signal which will be provided by the second digital processing means 2 . 4 to the operating means 2 . 5 .
  • the electronic noise discriminating threshold S_max i can be different from one reading electrode to the other in order to take gain and noise dispersions related to the detector and the electronics which follows the detector into account.
  • the amplitude discriminating means 20 can belong to the first processing means 2 . 3 and be located upstream of the digital processing means 2 . 20 if the first processing means 2 . 3 include digital processing means 2 . 20 or quite simply downstream of the analogue processing means 2 . 1 . Said analogue discriminating means 20 have been shown in FIG. 3B .
  • discriminating means are then analogue and applied to analogue pulses delivered by the analogue shaping circuit 2 . 11 or by the charge preamplifier 2 . 10 .
  • FIG. 5 shows as a diagram a digitized pulse processing algorithm S(i) implemented by the reading circuit and more particularly by the second processing means 2 . 4 which, in the example, are digital.
  • the first block referenced 60 corresponds to an electronic noise removal step.
  • the second processing means 2 . 4 include means 202 for determining one or more characteristic parameters of each digitized pulse S(i) being present downstream of the amplitude discriminating means 20 .
  • the means 202 for determining one or several characteristic parameters of each digitized pulse directly receive the digitized pulses from the first processing means 2 . 3 .
  • These characteristic parameters can be time parameters or a value reflecting an amplitude of the pulse digitized after a baseline crossing between the start and the end of the digitized pulse preserved.
  • the time characteristic parameter can for example be the rise time ⁇ T(i) of the preserved digitized pulse S(i).
  • the time parameter can be the time elapsed between the start of the digitized pulse and a first baseline crossing of the digitized pulse.
  • the value reflecting an amplitude of the digitized pulse after a baseline crossing between the start and the end of the preserved digitized pulse can be the minimum Min(i) of the preserved digitized pulse S(i), as will be explained later on.
  • the time parameter can be equal to the time variation between two discrete digital signals making up the digitized pulse, one of them being a discrete digital signal after the baseline crossing between the start and the end of the digitized pulse and the other discrete digital signal being for example the discrete digital signal illustrating the start of the digital pulse.
  • the processing algorithm illustrated in FIG. 5 which is only an example, only takes the minimum and rise time into account.
  • the minimum Min(i) corresponds to the minimum discrete digital signal of the digitized pulse S(i) from the reading electrode i. More precisely, this minimum Min(i) corresponds to the discrete digital signal having a maximum amplitude after the baseline crossing (that is herein after the zero crossing) of discrete digital signals making up the digitized pulse. Indeed, as already previously mentioned, in an induction share situation, the analogue pulse describes a first lobe, which is positive in the example studied, and then a second lobe, which is negative in the example studied, after a zero crossing.
  • the discrete digital signals making up digitized pulse S(i) coming from the analogue pulse also describe a first positive lobe, and then a second negative lobe after a zero crossing at the point P as illustrated in FIG. 4A .
  • the minimum discrete digital signal is the discrete digital signal having the highest amplitude in the negative lobe.
  • the minimum Min(i) corresponds to the discrete digital signal having a maximum amplitude after a zero crossing of the digitized pulse, this definition is applicable both when the first lobe of the digitized pulse is positive and when it is negative.
  • the rise time ⁇ T(i) corresponds, in principle, to the time interval separating the first discrete digital signal from the digitized pulse S(i) and the discrete digital signal having a maximum amplitude of the digitized pulse S(i).
  • the way of determining the rise time ⁇ T(i) has some importance in the processing efficiency.
  • the rise time ⁇ T(i) should depend as less as possible on the maximum amplitude of the digitized pulse S(i).
  • An efficient way to obtain a rise time ⁇ T(i) which does not take into account a maximum amplitude is to calculate a variation coefficient, or digital derivative, of the preserved digitized pulse S(i) coming from the electrode i, such a variation coefficient being positive.
  • the second digital processing means 2 . 4 are capable of derivating the digitized pulse S(i).
  • FIG. 4B represents the derivative of the digitized pulse of FIG. 4A , for the reading electrode i (curve C 3 ′) and for a neighbouring electrode, herein electrode i+1 (curve C 4 ′).
  • the second digital processing means 2 . 4 also include means 203 for rejecting any preserved digitized pulse which does not fulfil at least one induction share criterion, relating to one of the parameters previously determined.
  • the digitized pulse that fulfils this induction share criterion would be preserved.
  • rejecting means 203 cooperate with the means 202 for determining one or more characteristic parameters reflecting an induction share.
  • An induction share criterion relating to the minimum Min(i) could be:
  • S_min i being a predetermined minimal amplitude threshold.
  • a digitized pulse S(i) is due to an induction share, and thus that this should not be taken into account, when the minimum discrete digital signal, or more generally, when a discrete digital signal, of the digitized pulse S(i) exceeds a certain threshold S_min i after the baseline crossing of this pulse, the baseline being here represented by the zero value.
  • Thresholding can be performed by identifying the maximum amplitude below the baseline, previously defined by the term Min(i) and by comparing this maximum amplitude with the threshold S_min i . It can also be performed by rejecting a digitized pulse as soon as one of the signals crosses this threshold.
  • Digitized pulses which have a significant negative rebound are thus removed, this significant negative rebound being always present in the case of induction share as illustrated in FIG. 4A for the digitized pulse C 4 coming from the reading electrode i+1.
  • the digitized pulse C 3 coming from the electrode i would be preserved because it fulfils the induction share criterion relating to the minimum whereas the digitized pulse coming from the reading electrode i+1 would be rejected.
  • the absolute value of the rebound amplitude is higher than the amplitude of the positive part of the digitized pulse.
  • the value of the threshold S_min i can be empirically or experimentally determined.
  • An induction share criterion relating to the rise time ⁇ T(i) could be: ⁇ T ( i ) ⁇ S — ⁇ T i (2)
  • S_ ⁇ T i is a predetermined rise time threshold. Indeed, as can be seen in FIG. 4B , the rise time of the digitized pulse corresponding to an induction share is higher than the rise time of a digitized pulse corresponding to a collected charge.
  • the induction share criterion relating to the rise time enables digitized pulses, which have too small a rise time to correspond to the useful digitized pulse, to be removed.
  • the digitized pulse coming from the reading electrode i would be preserved because it fulfils the induction share criterion relating to the rise time, the digitized pulse coming from the reading electrode i+1 would be rejected.
  • the induction share criterion could be relating to the time variation, that is the slope, between two discrete digital signals making up the digitized pulse, for example between the start of the pulse and its maximum.
  • the criterion is such that the slope is compared to a certain predetermined threshold, and depending on the comparison result, the digitized pulse can be considered or not as being generated by an induction share.
  • the induction share criterion relating to the duration between two discrete digital signals of the digitized pulse is such that this duration is lower than a certain threshold, this reflects that the digitized pulse can be considered as being generated by an induction share. On the contrary, when the digitized pulse corresponds to a collected charge, this duration slowly tends to zero.
  • the second digital processing means 2 . 4 can apply only one of the criteria or combine several of them.
  • the second block 61 corresponds to one or several induction share removal steps.
  • the second digital processing means 2 . 4 can also include means 204 for determining digitized pulses S(i), S(i+1) coming from two neighbouring reading electrodes likely to reveal a charge share.
  • the lowest time acceptance window ⁇ possible will be selected, so as to restrict the fortuitous coincidences of two simultaneous interactions at neighbouring pixels. It can be a few microseconds or even less.
  • the second digital processing means 2 . 4 check whether the relationship below is fulfilled:
  • the second digital processing means 2 . 4 further include means 205 for rejecting one of both digitized pulses as revealing a charge share and for preserving the other. This rejecting means 205 cooperate with the means 205 for determining two preserved digitized pulses coming from two neighbouring reading electrodes.
  • the digitized pulse which is preserved is the one the maximum amplitude of which is the greatest. The other is rejected. It will be noted that the opposite would have been possible.
  • the second digital processing means 2 . 4 therefore apply the relationship (4) to select the digitized pulse to be preserved and to reject the other digitized pulse during the charge share processing: Max( i )>Max( i+ 1) (4)
  • the preserved digitized pulse S(i) is the one coming from the reading electrode i.
  • the rejected digitized pulse S(i+1) is therefore the one coming from the reading electrode i+1.
  • the second digital processing means 2 . 4 further include means 206 for correcting the digitized pulse S(i) selected with the rejected digitized pulse S(i+1). More precisely, the means 206 for correcting the digitized pulse selected add to the maximum amplitude of the selected digitized pulse the maximum amplitude of the rejected digitized pulse.
  • Max( i )corrected Max( i )+Max( i+ 1).
  • the selected digitized pulse is the one coming from the reading electrode i+1.
  • the rejected digitized pulse therefore is the one coming from the reading electrode i.
  • This corrected digitized pulse is a useful signal which will be operated and which is transmitted to the operating circuit 2 . 5 by the reading circuit 2 .
  • the third block 62 corresponds to the shared charge removal steps.
  • FIG. 3B an alternative of an ionizing radiation detection device object of the invention has been represented, in which the first processing means 2 . 3 are analogue and wherein the second processing means 2 . 4 are also analogue and are capable of correcting the induction share.
  • the first processing means have no digital processing means.
  • the first processing means 2 . 3 include as above a charge preamplifier 2 . 10 followed by a shaping means 2 . 11 . As already set out above, there could have been only the charge preamplifier.
  • amplitude discriminating means 20 can be provided as described previously.
  • the second processing means 2 . 4 connected at the output of the first processing means 2 . 3 include a cascade with a derivating filter 210 allowing the output of a signal which is a derivative of the analogue pulse delivered by the first processing means 2 . 3 , a time-amplitude converter 220 delivering a signal depending on the duration T during which the signal delivered by the derivating filter 210 is positive.
  • the signal delivered by the time-amplitude converter 220 is applied at the input of a comparator 230 , herein in this example on the non-inverting input of the comparator 230 .
  • This signal delivered by the time-amplitude comparator 220 will be compared to a predetermined set point time Tmin. This set point time is applied on the other input, herein in the inverting input of the comparator 230 .
  • the second processing means 2 . 4 further include a switch 240 a first terminal 240 . 1 of which is connected at the input of the second processing means 2 . 4 , that is at the output of the first processing means 2 . 3 and a second terminal 240 . 2 of which is connected to the operating means 2 . 5 .
  • This switch 240 is controlled by a signal delivered by the comparator 230 . The control is made such that the switch 240 is closed when the signal delivered by the time-amplitude converter 220 has a duration T higher than the set point time Tmin.
  • T the duration of the set point time Tmin.
  • the control is such that the switch 240 is open and the analogue pulse delivered by the first processing means 2 . 3 is rejected.
  • the means for determining a parameter are based on a time parameter which corresponds to the time elapsed between the start of the analogue pulse and a first zero crossing of the analogue pulse. This time is the one during which the analogue pulse has a positive amplitude.
  • the second analogue processing means could enable the rise time of the analogue pulse or the minimum of the analogue pulse after a zero crossing of the analogue pulse to be determined.
  • the implementation of such second analogue processing means is within reach of those skilled in the art.
  • Exemplary analogue pulses digitized by digital oscilloscope and coming from three neighbouring reading electrodes called i ⁇ 1, i and i+1 will now be considered referring to FIGS. 6A , 6 B and 6 C.
  • the detector is cadmium telluride CdTe detector.
  • the reading circuit 2 is in accordance with that illustrated in FIG. 3A .
  • the ionizing radiation source is a cobalt 57 gamma radiation source. It emits gamma photons the energy of which is about 122 keV.
  • Pulses represented in FIGS. 6A , 6 B, 6 C are delivered by a digital oscilloscope connected at the output of the analogue processing means 2 . 1 , upstream of the digital processing means 2 . 2 , that is upstream of the digitizer means 2 . 20 .
  • the place where digital oscilloscope would be connected is represented by arrows, in FIG. 3A .
  • the signal to be viewed is digitized beforehand by an integrated analogue-digital converter.
  • curves Ci ⁇ 1, Ci and Ci+1 respectively represent analogue pulses digitized by the digital oscilloscope and coming respectively from the electrodes i ⁇ 1, i, i+1.
  • curve Ci is the consequence of a gamma photon that interacted with the semi-conductor material associated with the reading electrode i.
  • Curves Ci ⁇ 1, Ci+1 illustrate that a low signal is induced on the neighbouring pixels associated with the electrodes i+1 and i+1.
  • curve Ci ⁇ 1 is the consequence of the interaction of a gamma photon with the semi-conductor material associated with the reading electrode i ⁇ 1.
  • Curve Ci has a significant amplitude, it corresponds to an induction share. It will however be noted that with the device according to the invention illustrated in FIG. 3A , if the minimal threshold S_min i is set to too low a value, for example equal to 0.05V, the digitized pulse coming from the electrode i will be taken into account and considered as a useful signal. The selection of the minimum threshold S_min i is to be made carefully.
  • FIGS. 7A and 7B show the energy spectrum provided by the reading circuit of an ionizing radiation detection device produced by a cobalt 57 gamma radiation source, this ionizing radiation detection device being provided with a cadmium telluride detector.
  • this spectrum illustrates the number of counts for each channel, a channel corresponding to a division of the pulse amplitude scale, for example a few hundred eVs. The number of counts is the number of pulses classified in each channel.
  • FIG. 7A the spectrum represented has been provided by a reading circuit in accordance with prior art and there has been no induction share removal, the induction share corresponding to the contribution of induced signals.
  • Pulses corresponding to induction share have a low energy amplitude and are visible for low energy channels, the number of which is lower than about 50.
  • the spectrum represented has been provided by a reading circuit in accordance with the invention having a function of processing, that is correcting, of the induction share.
  • FIG. 7C illustrates a case of share charge.
  • the sum of the maximum amplitude of curve Ci and the maximum amplitude of curve Ci ⁇ 1 substantially corresponds to the maximum amplitude of curve Ci of FIG. 7A .
  • the risk is to sum the amplitude of a pulse related to induction share with the amplitude of a pulse related to charge share. That is why it is more advantageous for the second processing means to first dispense with induction share before performing the correction related to charge share.
  • FIG. 8 are represented spectra still obtained with a same cadmium telluride semi-conductor detector and a same cobalt 57 gamma radiation source.
  • the spectrum referenced C 10 comes from digitized pulses that underwent no correction processing for induction share or charge share.
  • the spectrum referenced C 11 comes from digitized pulses that underwent the processing of charge share described above and the electronic noise removal processing.
  • the spectrum referenced C 12 comes from digitized pulses that underwent, in addition to the electronic noise removal processing, first the processing of induction share and then the processing of charge share.
  • Spectrum C 12 is such that its peak corresponds to digitized pulses coming from neighbouring electrodes for which a maximum has been summed, neither of both digitized pulses which have been summed has undergone a processing of induction share.
  • spectrum C 12 it can be said that the number of counts with a low amplitude has been reduced, in channels the number of which is lower than about 40, with respect to spectrum C 11 or C 10 , since induction share has been dispensed with.
  • the number of counts in intermediate channels has also been reduced, the number of which is between about 40 and 120, with respect to spectrum C 10 since charge share has been dispensed with.
  • the number of counts has slightly increased in the peak of spectrum C 12 , for channels the number of which is between about 120 and 170 without degrading too much the energy resolution since the processing of charge share has been performed after the processing of induction share.
  • pulses to dispense induction share and possibly charge share are only possible because said pulses have been continuously digitized at the output of the first processing means.
  • energy detection threshold it is meant the minimum detectable energy. Indeed, the signal obtained in low energy channels is not, or is less, saturated with signals corresponding to induced signals. It rather represents interactions that release a low energy.

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US10098595B2 (en) * 2015-08-06 2018-10-16 Texas Instruments Incorporated Low power photon counting system
US10627531B2 (en) * 2016-04-14 2020-04-21 Commissariat à l'énergie atomique et aux énergies alternatives Device for counting particles for a radiation detector
US20190250288A1 (en) * 2016-10-27 2019-08-15 Detection Technology Sas Spectrometry device
US10890676B2 (en) * 2016-10-27 2021-01-12 Detection Technology Sas Spectrometry device
US10151845B1 (en) 2017-08-02 2018-12-11 Texas Instruments Incorporated Configurable analog-to-digital converter and processing for photon counting
US10481283B2 (en) 2017-08-02 2019-11-19 Texas Instruments Incorporated Configurable analog-to-digital converter and processing for photon counting
US10024979B1 (en) 2017-11-01 2018-07-17 Texas Instruments Incorporated Photon counting with coincidence detection
US10890674B2 (en) 2019-01-15 2021-01-12 Texas Instruments Incorporated Dynamic noise shaping in a photon counting system
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